Self-synchronisation of wind farm in MMC-based HVDC system

Abstract

Abstract-The stability of an offshore wind power network connected through a high voltage dc (HVDC) transmission line is a critical problem since there is no direct connection to a strong ac collection (ACC) bus. Field experience has shown that subsynchronous oscillation (SSO) and harmonic resonance can occur between the wind farms and the HVDC systems. The oscillations can appear in the presence of background harmonics and is arguably resulting from the controller interaction of the Wind Energy Conversion System (WECS) converter controller and the HVDC converter controller. The design of the synchronization unit (Phase-Locked-Loop) has shown to have a significant impact in achieving satisfactory performance. However, both slow and very fast synchronization units could directly affect the control performance and degrade the system stability. This paper proposes a controller design without a Phase-Locked-Loop (PLL) for the WECS grid-side converter which does not have a dedicated synchronization unit (PLL) in the controller. This controller is implemented on the WECS converters of the ACC side to synchronize them to the Modular Multi-level Converter (MMC)-based HVDC system. A detailed analysis is presented and the results are compared with the widely used decouple dq-frame control structure. The impedance frequency responses for both the dq-frame control and the synchronverter-based control are presented to show a comparison of the system performance. Time domain simulation results are presented to show how the selfsynchronisation impacts on the system performance compared to the classical dq-frame control solution.

Sub‐synchronous oscillation mechanism and its suppression in MMC‐based HVDC connected wind farms

“…Several papers have so far discussed the stability of MMC-HVDC connected wind farms [9][10][11][12][13][14]. The SSO phenomenon in the MMC-HVDC system with wind farms was reported in [9,10], in which the propagation mechanism of the SSO current in the MMC-HVDC system was investigated.…”

“…The stabilization control methods for enhancing the stability of the MMC-HVDC based wind farms were investigated in [13], whereas the factors that influence the stability of the interconnected system were not discussed. Additionally, the small-signal stability of synchronverter based wind farm with MMC-HVDC was investigated in [14], which shows the synchronverter's superiority in grid-integration stability in comparison to the PLL-based dq-domain current control, but again the paper merely focused on the control and stability mechanism of the wind power inverter while neglecting the MMC internal dynamics. In addition, the high-frequency resonance issues caused by the distributed capacitance of the long transmission cables [15], the parallel resonance in wind farms caused by the shunt capacitor banks and high-frequency harmonic filters [16], and the subsynchronous resonance in DFIG-based wind farms caused by the series-compensated capacitors [17] were also investigated.…”

Synchronverter: A Comprehensive Review of Modifications, Stability Assessment, Applications and Future Perspectives

“…Above 2 kHz, the inductive characteristics have been exhibited. However, below 2 kHz, the vector control has multiple resonant peaks which could lead to oscillatory response and voltage instability [22], [23].…”

“…The absence of PLL and cascaded loops in synchronverter can reduce the potential interaction between them. Hence synchronverter has been used to mitigate the sub-synchronous oscillations and improve the stability margin of the off-shore WES and HVDC system [22], [23]. Similarly, the synchronverter based HVDC has been used to enhance power transfer capacity between interconnected weak AC systems.…”

Oscillation Mechanism and Mitigation of MMC-HVDC Based on Bifurcation Theory for Wind Farm Integration

“…The transmission of the large-scale centrally developed wind power through MMC-HVDC has become the trend of technique development, and several projects have been put into operation or under planning and construction [4]- [6]. But with the continuous increase of the installed proportion of wind power in some areas, the connected synchronous power grid is gradually weakened, the stability characteristics of the power system have profoundly changed, and the broadband oscillation phenomenon of large-scale integrated wind power occurs frequently, which critically affects the security and stability of power grid operation [7]- [10].…”